1. Field of the Invention
The present invention relates to a vibratory flowmeter, and more particularly, to a dual pick-off vibratory flowmeter.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pick-off sensors and processes the signals in order to derive a mass flow rate measurement.
Vibratory flow meters are often used for measuring the mass flow rates of flow fluids at relatively low pressures. However, there is a need for mass flow measurement at very high fluid pressures. Under very high fluid pressure conditions, the flow conduits and other flow handling equipment must by constructed to be heavy and strong.
However, such structural strength makes vibration of a flow meter assembly problematic, as a high-pressure flow meter assembly will be substantially rigid. A high-pressure flow meter assembly also makes measuring a response to the vibration hard to quantify, as the vibrational response will be relatively small due to the rigidity. Additionally, the flowtube vibrational frequency will not be highly affected by the fluid density of the flow material due to the ratio of structural mass to non-structural mass, i.e., the mass of the flow fluid is much less than the mass of the flow conduits. With small changes in frequency, the resulting vibrational amplitude and therefore the resulting pick-off voltage will be at a minimum, resulting in a low meter sensitivity. Discrimination of a relatively small pick-off voltage will be more difficult, as the signal-to-noise ratio will be minimal.